Magnetic Anisotropy Energy Barrier Distribution and Surface Magnetism in CoPt Nanoparticle

The magnetization of the protein shell (PepA) encapsulated CoPt nanoparticle (average diameter = 2.1 nm) specimen was investigated by implementing non-equilibrium magnetization calculation scheme and Curie-Weiss type formula. In doing that, the magnetization measurement sequence of SQUID (Superconducting Quantum Interference Device) magnetometer was modified to comply with the numerical analysis scheme. Applying the analysis methodology to the CoPt nanoparticles, we extract the values of zero-temperature coercive field and Bloch coefficient, which are in good agreement with the previously reported values. Remarkably, the distribution of the zero-field zero-temperature anisotropy energy barrier follows an exponential type function, which is uncorrelated with the known size distribution of the particles. The lacking correlation between them was reported in ferritin, suggesting that it is a generic feature of certain magnetic nanoparticle systems having a range of disorders and imperfections. In addition, we analyze non-saturating behavior of a field dependent magnetization curve in terms of Curie-Weiss type formula, allowing a separation of the surface magnetic moments from the core moments.